The support of large underground and civil excavations may require the use of any combination of “rockbolts”, dowels and/or cables. For more permanent support where the support duties exceed the capacity of traditional “rockbolts and dowels”, cable reinforcement is used. Where groundwater or corrosive conditions are present, support regimes using cables or rockbolts in conjunction with a cement or resin grout are used.
The properties of grouts, such as deformation modulus, uniaxial compressive strength and shear strength under normal stress, are variable according to the water/cement ratio of the grout. Typically, the ideal water/cement ratio of grouts for use with cable reinforcement lies in the range 0.30 to 0.35. Grout may be introduced into a cable bolt hole by using varying methods including the “collar-to-toe method” or the “toe-to-collar method”.
In relation to the “collar-to-toe method”, grout is injected into the entrance of the hole through a small diameter tube. A tube of similar diameter, known as a breather tube, extends to the other end of the hole alongside the accompanying cable. As grout travels through the hole, air is bled through the breather tube. When the hole is full of grout, the grout travels through the breather tube and back out the entrance of the hole.
In the “toe-to-collar method”, a small diameter grout injection tube is pushed to the end of the hole and alongside the accompanying cable. The tube is then slowly displaced as the grout is pumped into the hole. This method does not require the use of a breather tube.
It is widely recognised that it is difficult to pump a high quality of grout into the hole using either of the two abovementioned methods. As a compromise, less viscous grouts are pumped into the holes which results in several unfortunate consequences. Firstly, a less viscous grout does not readily remain in the hole, resulting in lost grout through the hole entrance. Secondly, a high water/cement ratio grout reduces the overall capacity of a grouted cable. A further consequence of grout running out of the entrance of the cable hole is the increase in potential for spillage of grout onto personnel and equipment.
Excess grout may be prevented from running from the hole entrance by several plugging techniques and combinations thereof, including insertion of cotton wadding to fill gaps between the cables, tubes and hole; spraying an expanding foam into the hole entrance; and insertion of a wooden spad or plug in the hole entrance to jam the tubes and cable as tightly together as possible to minimise gaps. Alternatively, one must rely on a perfect, thixotropic grout to remain in situ.
The plugging methods described above have several limitations. The technique used to insert cotton wadding is slow and messy, and gaps may still remain around the cables and tubes from which grout may escape. Further to this, a small pressure buildup in the hole may cause the cotton wadding to be ejected out of the hole during the grouting process causing grout to spill out of the hole entrance. The hole then requires “replugging”. Cotton wadding is easily wasted and is useless once it becomes wet or soggy. Foam sprays are generally very expensive and require a “curing period” before cables can be grouted, thus adding a further step in the whole procedure. Foams require special handling (e.g. use of gloves to avoid contact with the skin) and may also produce toxic fumes and are not recommended in areas of minimum ventilation. Further to this, foam sprays are very messy and often result in wastage. Wooden spads or plugs do not generally provide a tight seal.
The present invention attempts to overcome at least in part some of the aforementioned disadvantages.